The Athletics struggled on and off the field in Kansas City, although surrounded by a carnival-like atmosphere. Among his famous stunts were a mechanical rabbit that would pop out of the ground near home plate to deliver fresh baseballs to the umpire, a live mule (also named Charlie O.) that served as the team's mascot, or an attempt to introduce colored baseballs. When the relationship with the city became untenable, he moved the A's to Oakland for the 1968 season.

He had his teams wear garish uniforms in gold and green, and attempted to dump the venerable "Athletics" nickname in favor of the more-modern sounding "A's". The American League had to promise the city of Kansas City, MO an expansion franchise to keep from being sued after Finley decamped. Finley found success in Oakland, though. He was among the first to understand the importance of the newly-created amateur draft and used it to build a lineup of young stars such as Reggie Jackson and Catfish Hunter, soon making the club a contender. The A's won the AL West for five consecutive seasons, from 1971 to 1975, and they captured three straight World Series from 1972 to 1974. The teams were also notable for wearing mustaches, something done at Finley's insistence to enhance the A's image. During the 1973 World Series, he attempted to fire 2B Mike Andrews after he committed two critical errors in Game 2. Finley actually tried to put Andrews on the disabled list so he could bring in a replacement, but CommissionerBowie Kuhn ordered Andrews reinstated. Disgusted by Finley's move, manager Dick Williams quit immediately after the World Series ended.

Finley was convinced that speed was one of the keys to winning ballgames and forced his teams to carry a full-time pinch-runner throughout the 1970s. He used a track star with no baseball experience - Herb Washington - in the role during the 1974 and 1975 seasons, but although the sprinter had great speed, his lack of baseball instincts were a problem. Others used in the role like Allan Lewis, Don Hopkins, Matt Alexander or Larry Lintz were genuine baseball players, albeit one-dimensional.

However, in the era of free agency, Finley struggled to compete. He lost Hunter to free agency after the 1974 season because he had failed to honor a clause in his contract, and was forced to trade Jackson to avoid facing the same fate. In 1976, he attempted to sell outfielder Joe Rudi and pitchers Rollie Fingers and Vida Blue for large sums of money, but commissioner Bowie Kuhn stopped the deals, claiming they would be detrimental to the game. The first two left as free agents after the season, along with other key contributors to the three World Series titles, such as Bert Campaneris and Sal Bando, while he traded Blue to the San Francisco Giants for six young players just before the start of the 1978 season in order to keep the team afloat. It didn't work, as they stayed in the cellar. He eventually sold the team in 1981, after attempting unsuccessfully to have the team sold and moved to Denver, CO. He tried to run the club on a shoestring budget, even declining to have the team's games broadcast over the radio, to lower attendance and force acceptance of a move. He was successful in the first, but not in the second.

Much like George Steinbrenner, Finley had a reputation for changing managers often. In the 20 seasons he ran the club, the A's had 15 different managers. His departure from the game was generally greeted by a sigh of relief, although he saw himself as a visionary who had been unfairly treated by Bowie Kuhn and his fellow owners. He was inducted into the Indiana Baseball Hall of Fame in 1980.